Integrated grating-outcoupled surface-emitting lasers

ABSTRACT

An electronic/photonic integrated circuit in which grating-outcoupled surface-emitting lasers are used both to provide external emission out of the plane of the chip (through gratings), and also to feed optical power into photonic waveguides parallel to the plane of the chip. Transistors are fabricated in a common multilayer semiconductor body with the lasers.

CROSS-REFERENCE TO OTHER APPLICATION

This application claims priority from 60/230,534, filed Sep. 1, 2000,which is hereby incorporated by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to Electronic/Photonic IntegratedCircuits, in which optical emitters are integrated with complexintegrated circuitry.

Background: Integration of Emissive Electro-Optics

Vast advances have been made in integrated circuit electronics over thelast few decades. At the same time, the technology of solid-state lasershas advanced greatly, and such lasers have proven useful in manyapplications. However, there has been no successful merger of thesetechnologies, and the two have continued to develop along generallyseparate paths.

Imagers are inherently much simpler to integrate than emissive optics.(Indeed, in the last few years CMOS imagers, which use basicallymainstream CMOS technology, have largely displaced even CCD imagers.)Many chips and hybrids have combined an imaging array with drivers orother associated circuitry.

Emissive optics, however, have been much more intractable tointegration. Various attempts have been made to propose a technologywhich would combine emissive optics with complex integrated electronics,but no such proposal has come remotely close to practicability.

Thus while there is a great need for a fully integrated emissiveElectro-Photonic Integrated Circuit (“EmEO”) technology, there has beenno adequate solution to this need.

Background: GSE Lasers

The Grating-Outcoupled Surface-Emitting (GSE) laser (described incommonly assigned U.S. patent application Ser. Nos. 09/844,484 and09/845,029, both of which are hereby incorporated by reference), is anessentially planar structure which provides out-of-plane opticalemission. The GSE laser has a built in horizontal waveguide that allowson-wafer or on-chip routing and control of light along with emissionfrom the surface of the wafer or chip. In contrast, the light fromvertical cavity surface-emitting lasers (VCSELs) is directed normal tothe wafer or chip surface and cannot easily be routed within the waferor chip. The epitaxial structure of a VCSEL is very thick and thereforecostly and time consuming to grow, compared to the relatively thinlayers making up an edge-emitting (EE) or GSE laser. While EE lasershave a horizontal waveguide and can route light within a wafer or chip,at least one terminating edge (cleaved or etched) is required to accessor connect the on-chip light to the outside world. Thus EE lasers areinherently edge-bound (and hence not fully integrable), while VCSELshave incompatibility due to their very special epitaxy requirements.

Integrated Grating-Outcoupled Surface-Emitting Lasers

The present inventors have realized that GSE laser technology providesthe foundations for an economical and fully integrable emitterstructure, and for a new Emissive Electro-Optic Integrated Circuittechnology in which optical emitters (and possibly other opticalcomponents) are integrated with complex integrated circuitry. Thisprovides a technology in which on-chip photonic signal channels arecombined with unconstrained location of photonic output or inputcouplers to the outside world. Preferably distributed reflectors areused to define the on-chip laser cavities, so that the locations oflaser cavities are not tied to wafer edge or facet locations.

It is highly preferred, in many embodiments, that the optical gainvolume (of the emissive photonic elements) and theconductivity-modulation volume (of the active electronic elements)should be formed using a SINGLE body of semiconductor material. Manyapproaches have been proposed for hybrid structures and for modules, butall have proven to present substantial technological difficulties.

The disclosed innovations, in various embodiments, provide one or moreof at least the following advantages:

capability for on-chip routing of optical signals;

combined capability for both on-chip routing and third-dimensionaloutcoupling of optical signals;

the manufacturing difficulties of faceted wafers are avoided; and

the expensive epitaxy of vertical-cavity lasers is avoided.

BRIEF DESCRIPTION OF THE DRAWING

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

FIG. 1a shows side view of a GSO+DBR laser, and FIG. 1b shows a topview.

FIG. 2a shows a side view of crossed DBR lasers, and FIG. 2b shows a topview.

FIG. 2c shows a close up of crossed outcoupling gratings.

FIG. 3 shows a side view of a DBR laser with a reflective undercoatingto reflect laser light.

FIG. 4a shows a top view of a DBR with flared or tapering gain regions,and FIG. 4b shows a top view of crossed DBRs each with flared or taperedgain regions.

FIG. 5a shows a side view of a laser diode having a DBR at one end and acleaved facet at the other end.

FIG. 5b shows a top view of crossed lasers, using both DBRs andreflecting facets.

FIG. 5c shows a top view of a laser diode using cleaved facets.

FIG. 6 shows a laser diode using cleaved facets and a reflective layerbeneath part of the waveguide.

FIG. 7a shows a top view of four crossed DBR lasers each outcouplinglight through the same outcoupling element.

FIG. 7b shows a close up of the crossed gratings for the laser system ofFIG. 7a.

FIG. 8 shows a circuit diagram of integrated elements with the presentlydisclosed laser system.

FIG. 9 shows optical waveguides routing light from the laser to otherelements.

FIG. 10 shows a possible configuration for integrated elements with alaser diode.

FIG. 11 shows another possible configuration for integrating addedelements to the present innovations.

FIG. 12 shows another embodiment of the present application.

FIG. 13 shows another embodiment of the present application.

FIG. 14 shows a DBR laser with thinned quantum wells beneath theoutcoupling grating and beneath the DBRs.

FIG. 15 shows another laser embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferredembodiment (by way of example, and not of limitation).

FIG. 8 shows an example of a higher level circuit integration on thesame chip as the grating outcoupled laser diode. In this example, laserdriver and control electronics are integrated. Din and Din-bar aredifferential data inputs and EN is the chip enable. Internal logic andBIAS control is used to center the data to the inputs of the driver pairand to provide a Vref output. This output is used with Rset to controlthe current source or modulation current Imod which sets the outputcurrent to the grating outcoupled laser diode. The resistor Rext isexternal to minimize the power dissipation capacity, both Rext and Rsetmay be integrated as well.

FIG. 9 shows the optical waveguide technology routing the light from thelaser source to another integrated circuit component, for example. Oneend of the laser has a partially transmitting reflector 902 which allowssome of the light to pass through. This light is guided by a waveguide904 to another circuit element as desired for any given application. Inthe example shown, a corner turning mirror 906 is used to guide thelight.

Other devices can also be integrated on the same chip with theinnovative laser system of the present application. FIG. 10 shows agrating outcoupled laser diode with the OCG 8 between the two reflectors902 at either end of the cavity. One or both reflectors are partiallytransmitting gratings in this example. In this variation, a waveguidecoupler, which may be used for connecting to other integrated devices,is located outside the laser cavity. A location for such an integrateddevice 1002 is shown at either end of the cavity. Of course, otherdevices could also be integrated, for example, modulators or tuningsections.

FIG. 11 shows another possible variation for integrating addedcomponents (such as a phase adjustor, for example). In this example, tworegions 1002 where added devices may be fabricated on chip are shown. Inthis example, the added components are located within the length of thelaser, possibly requiring the gain region contacts to be moved oraltered to accommodate the added device.

Details and Alternatives for Laser Design and Fabrication

Preferred implementations of photonic elements will now be described, aswell as a wide variety of modifications.

First order outcoupling gratings and second order or higher outcouplinggratings are both used in at least some embodiments of the presentinnovations. In the present application, first order DBR refers to adistributed Bragg reflector grating that reflects light within thewaveguide in first order for feedback. A second order DBR grating willoutcouple light in first order, and feedback light in second order.

In several variations in this application, second order feedbackgratings (which couple light out in first order ) are used. In sucharrangements, the feedback grating depth or strength is varied in the yand z directions so that both the loss and the feedback from the gratinghelp to stabilize the laser mode. For example, the first order lateralmode will be stabilized if the grating strength is varied so that thefeed back varies like a Gaussian. This is accomplished by forming thegrating so that its strength varies as

1−exp[−(y/ω)²]

where y is the direction parallel with the feedback grating surface andperpendicular to the cavity length, with the origin taken to be at thecenter of the out-coupling grating, and omega is half the y gratingdimension.

First order outcoupling gratings are gratings which couple light out ofthe waveguide plane in first order but may or may not satisfy thein-plane Bragg condition for second or higher order Bragg reflection.Such gratings may be designed to create no second or higher orderreflections which feedback into the laser mode. In these variationswhich use such out-coupling gratings with no in-plane feedback, thegratings cause no destabilizing feedback into the laser mode and aremechanically and electrically isolated from the structure used to formand pump the resonant laser cavity. Thus, the length and position of theoutput grating can be chosen to suit the needs of the application forwhich the laser is designed. The grating periods required foroutcoupling, with and without in-plane reflections, are summarized in“Surface Emitting Semiconductor Lasers and Arrays,” G. A. Evans and J.M. Hammer, Eds., Academic Press, 1993, which is hereby incorporated byreference.

In general, second and higher order feedback gratings can result in someoutcoupling. However, these are less preferred in the context of thepresent application since such higher order interactions are lessefficient.

The outcoupling angle of the gratings in the innovative systems hereindisclosed is measured as an angle from the normal to the surface of theoutcoupling grating. Resonant outcoupling occurs when the outcouplinggrating has a period that is equal to an integer number of wavelengthsof the light in the cavity. A grating with period equal to thewavelength of light in the laser cavity will outcouple some light normalto the laser plane and reflect some light in-plane in second order. Thismeans the light exits the grating parallel or nearly parallel to thenormal. Outcoupling of light off the normal occurs when the grating isnot an integer number of guide wavelengths, and in such a case the lightexits the grating at an angle from the normal. This angle depends on thedifference between the guide wavelength and the grating period. Varyingthe wavelength of light or the outcoupling grating period can thereforehave great effect on the angle of outcoupled light. The out-couplinggrating length, longitudinal position, and the output angles maytherefore be chosen over a large range of values. The grating may alsobe shaped to achieve an output beam of a desired cross section. This isvaluable for coupling the output light into fibers of different crosssectional size or at different angles than exactly or nearly normal. Allof these “off normal” parameters may be varied without fear ofsignificant mode destabilization or disruption of laser coherence. Inthe case of exactly or near normal outcoupling, there can be significantin-plane reflection that may (or may not) adversely affect theperformance of the laser.

FIG. 1a shows a cross sectional view of a preferred embodiment, taken toshow the x-z plane. It should be understood that the features in theseveral figures may not be to exact scale because of the largedifferences in dimension between the various structures illustrated.

Layers 3, 4, 5, and 6 are grown on a substrate 2 by known means. Each ofthese layers may comprise a number of sub-layers. Beneath the substrateis the n contact layer 14. The substrate may comprise a thick layer ofn-type semiconductor with a top layer of similar n-type materialinterposed beneath layer 3. This is frequently called the n-cladding orn-clad. The n-clad will have a refractive index below that of layer 3.Layer 3 is the active and guiding layer usually containing the junctionbetween p- and n-type semiconductor materials. It may comprise, forexample, a sequence of one or more barrier layers alternating withquantum well layers. Layer 4 is a p-type clad layer and has lowerrefractive index than layer 3. Layer 5 may be a multi-layer including ap-clad material chosen to enable good contact to 6 which is thep-metallic contact. Layer 14 provides the other electrical contact forthe laser. There are many sequences of possible layers for semiconductorlasers and amplifiers, and the present innovations are not limited tothe structures recited here. For example, a structure with a p-typerather than an n-type substrate (and all the necessary alterations toaccommodate a change from p- to n-type materials and vice versa) iswithin the contemplation of the present application.

Gratings 7 are surface relief DBR gratings chosen to reflect light inthe +/−z direction to form the laser cavity. (Note that these gratingscan be buried structures within the device, and the term “surfacerelief” does not require the grating be on the surface of the deviceafter processing.) The laser mode will be a standing wave which may beconsidered to be formed by two waves one flowing in the +z direction,the other in the −z direction. First order DBR gratings are preferred,but second or higher order gratings are also possible. The DBR gratingdepth and length and the thickness of layer 4 are chosen to provide thedesired feedback as known in the art.

The reflector gratings can be given added functionality by varying theirgrating strength or amplitude in both the y (lateral) direction and thez (cavity) direction. Variation of the grating strength in the lateraldirection will impart to the cavity mode light a Gaussian shape,allowing for more of the optical energy of the emitted light to becoupled into a circular mode, such as a fiber. Variation of the gratingstrength in the z direction can improve the suppression of unwantedlongitudinal modes on either side of the desired longitudinal mode. (Thedegree to which the unwanted modes are suppressed is called theside-mode suppression ratio.)

The outcoupling grating 8 (sometimes referred to herein as OC grating,or OCG) is a surface relief grating with period chosen to couple lightat desired angles from the grating plane. It is located at an apertureon the surface of the device. In a preferred embodiment, the outcouplinggratings are about 10 microns wide. The outcoupling grating may beshaped to control the shape of the emitted beam. The grating depth andthickness of the p-clad layer 9 in the vicinity of the grating 8 arechosen to provide the desired degree of outcoupling and to control beamshape. A window or aperture 10 in layers 5 and 6 is provided to allowunobstructed emission of light, and the size and shape of theoutcoupling grating is matched to the mode of the fiber to which itcouples light (in one embodiment). Because of the two standing waves inthe cavity and reflection from the grating, the outcoupling gratingsimultaneously emits four different light beams, two above and two belowthe grating plane. These are depicted by dashed arrows. In the case ofnormal outcoupling of the laser light, the two top lobes are combinedinto a single beam, as are the two bottom lobes of emitted light.

In one embodiment, the outcoupled light is emitted normal to thesurface, since one primary goal is to couple this light into a fiber.When light is emitted normal to the surface, the two top emitted beamsare combined into a single beam, and likewise with the downward emittedbeams.

Off normal emissions and slightly off normal emissions are also veryuseful. For example, changing the angle of entry to a fiber by severaldegrees has minimal impact on the coupling efficiency yet allows the useof an off resonance grating which minimizes undesired feedback into thelaser. A larger angle might be desirable to send light to anotherdetector to monitor the laser.

FIG. 1b shows a top view of a single grating outcoupled DBR laser. Theoutcoupling grating 8 is located at an outcoupling aperture within theenvelop of the gain region. On either end of the laser are locateddistributed Bragg reflectors 7 for providing feedback into the cavity.Of course, cleaved facets may also be used instead of reflectorgratings, with highly reflective coatings applied to reflect the light,as shown in later embodiments. With either DBR reflectors or coatedfacets, the reflectivity of one or both ends can be varied to allowlight to escape the cavity for other purposes.

Another embodiment will be discussed with reference to FIGS. 2a and 2 b.In this variation, crossed out-coupling gratings are located within thecavities of two (or more) semiconductor lasers arranged at angles to oneanother and located on a common substrate. In one embodiment, two lasersare used and are positioned at 90 degrees from one another, but morelasers are of course possible—see FIG. 7a for example. The shape andstrength of the two gratings are chosen to produce desirable propertiesin the out-coupled light. Their periods are individually chosen to suitthe desired application, such as to control outcoupling angle, or tocouple out different wavelengths.

FIG. 2a shows a side view of the crossed grating outcoupled DBR lasers.The structure when seen from the side is similar to that of FIG. 1.Elements that are unique to the laser running in the z-direction arelabeled with a z suffix, and elements unique to the laser running in they-direction are labeled with a y suffix.

Referring to FIG. 2b, a top view, two crossed DBR lasers are at 90degrees to one another. Each laser has its own set of reflector gratings7 y, 7 z at either end, and both lasers have their own out-couplinggrating 8 y, 8 z positioned at a common location between the reflectorgratings. (In the preferred embodiment, the outcoupling aperture islocated at the center of the laser, but this is not necessary.) Oneither side of the out-coupling gratings are the pumped regions of thelasers. (Note that in this variation, the two gain regions of a singlelaser are discontinuous, having different parts on either side of theoutcoupling grating. Other possible embodiments include a single gainregion with an outcoupling grating outside the gain region but betweenthe reflector gratings, or even a single continuous gain region thatspans the outcoupling grating, having portions on both sides.) The twoout-coupling gratings are located at the same place, and thesuperposition of the two gratings forms a virtual grating with aneffective period at an angle of about 45 degrees if the grating periodsare about the same fore each laser.

The reflector grating periods are chosen to internally reflect theproper wavelength of light. The reflectivity of the DBR is very high atthe Bragg condition, and drops off rapidly when the wavelength isdifferent than the Bragg condition wavelength. This allows thewavelength of the output light to be controlled by controlling theperiod of the DBR gratings.

Referring again to FIG. 2a, in the case of crossed lasers, a crosssection taken parallel to the x-y plane would be similar with layersnoted with y subscripts in place of z subscripts.

Gratings 8 y and 8 z are surface relief outcoupling gratings withperiods chosen to couple light at desired angles relative to the grating(y-z) plane. As shown in the figure, the gratings can be shaped tocontrol the profile of the outcoupled beam. A circular profile for thegrating is indicated (a more complicated profile would be optimal forfiber coupling), but any other useful shape can be produced, dependingon the application. The grating depth along with the thicknesses andcompositions of the epitaxial structure of the laser are chosen toprovide the desired degree of out-coupling.

For each laser, four beams are emitted because of the left and rightrunning waves that form the standing wave mode of the laser (unless thelight is outcoupled perpendicular to the device). Two beamssymmetrically angled around the normal will emerge above the gratingplane and pass through the window 10. Similarly, two beams will bedirected towards the substrate below. (Note the epi layers aretransparent and this transparency can be made use of to couple light outthrough the bottom of the device. In such a case, a reflector is placedon the top of the device to direct the top emitted light back into thelaser or out the bottom.) In some designs, the grating may be blazed toallow light to be outcoupled to the right or left of normal as well.

When two or more lasers are combined in this way, the crossedoutcoupling gratings each polarize the light which they outcouple. Inthe case of two crossed gratings at 90 degrees, the two beams of lightwill be polarized at 90 degrees with respect to one another. Couplinglight into a fiber with two orthogonal polarizations is required forpumping Raman amplifiers.

FIG. 2c shows a close up of the outcoupling gratings of FIGS. 2a and 2b. The periods, Ly and Lz, of the superimposed two gratings need not beidentical. The OCG periods will depend both on the wavelength of lightin the cavity (which in turn depends on the periods of the DBR gratingsat either end of the cavity) and on the desired outcoupling angle forthe emitted beam. By choosing the two gratings to have different OCangles, spatial separation is possible, as may be desired by particularapplications. In still another embodiment, the laser light from thedevices is emitted normal to the surface, so that both wavelengths oflight can be coupled into a fiber through the same aperture or locationon the device.

By choosing a non-resonant spacing for the outcoupling grating period(i.e., a distance between grating lines that is not an integer multipleof the wavelength of light within the cavity) the output beams areemitted non-normal to the surface. This is useful in applications where,for example, the fiber into which the light is to be coupled is at anangle relative to the out-coupling grating.

The choice of normal or off normal outcoupling angles can have otheradvantages. For example, when two or more different wavelengths of lightare coupled out of the OC gratings, all wavelengths can be coupled intothe same fiber or separated as desired by varying the output angles ofthe individual gratings. For example, the individual outcouplinggratings for two crossed devices could each couple different wavelengthsof light out normal to the surface to couple two different wavelengthsinto a single fiber mode. This is particularly applicable in the crossedgrating out-coupled lasers, discussed further below. If, for somereason, only one wavelength is needed in the fiber, the light from theother device can be emitted off normal so as to not couple into thefiber. The non fiber coupled light could be deflected to a detector, forexample. Regardless of the particular use, the choice of outcouplingangles is an advantage to a laser device, and the present applicationshows how different wavelengths from different sources can beselectively combined into a single region for coupling, or separatedinto different regions.

The shape and strength (i.e., the depth) of the OC gratings are chosento produce desirable properties in the out-coupled light. The periods ofall OC gratings can be individually chosen according to the needs ofthat particular laser and the application. For example, the two crossedOC gratings of FIG. 2b can be chosen to outcouple different wavelengthsof light, allowing the two lasers of the crossed laser configuration tohave different wavelengths, one in the z-direction, another in they-direction. This of course extrapolates to higher numbers of lasers.Additionally, the two outcoupling gratings (and the different laserdiodes themselves) can be chosen to emit the same wavelengths (forexample, by making their feedback grating periods the same) allowingadditional power and polarization variety in the outcoupled beam(s).

The basic idea can be extended to include a multiplicity of lasersradially arranged around a set of gratings oriented to outcouple lightindependently for each laser. This allows many wavelengths of light tobe generated by merely choosing a different period for the pair of DBRsfor each individual laser. The OC gratings can couple this light into asingle spatial region (for example, to couple several wavelengths oflight into a fiber for DWDM applications), or it can couple thedifferent wavelengths out of the devices at different angles.

Referring still to FIG. 2b, which shows crossed lasers according to apreferred embodiment, if the Bragg reflector gratings are chosen to havethe same period in both the y-direction laser and the z-direction laserso that both lasers operate at the same wavelength, and if the crossedOC grating period Lz is the same as Ly, the superposition of the twogratings at right angles results in a virtual grating with an effectiveperiod angle of about 45 degrees (if both grating periods are the same).In this case the possible coupling between the y and z lasers can beavoided if the gain regions use quantum wells with compressive strainand thus favor TE mode operation. The virtual grating at 45 degrees willnot efficiently reflect TE modes and therefore will not couple the y andz lasers. On the other hand, the use of tensile strain in the quantumwell favors TM modes, and may result in enough coupling to either lockthe y and z lasers together into a single coherent source, or providesignificant cross-talk and other interactions between the two lasers.Thus, the disclosed approach can choose the nature of the output beamsto be either a combined single frequency coherent source, or two outputbeams with two independent wavelengths, or two beams with independentwavelengths but with a controlled amount of crosstalk between them.

The reflector grating periods for the pair of lasers can be the same,which provides additional power and polarization variety. Alternatively,the grating periods can be different, resulting in two differentwavelengths of light being outcoupled. This latter configuration cancouple light of different wavelengths out at the same angle for couplinglight of different wavelengths into the same fiber, saving the cost ofimplementing a combiner for this function. For example, if the twolasers have different feedback grating periods, they will each generatea different wavelength of light. But both lasers can emit their lightnormal to the surface of their respective outcoupling grating bychoosing each individual outcoupling grating to couple the necessarywavelength of light out normal to the surface.

The size of the grating output aperture can be adjusted for optimumcoupling to single or to multi-mode fiber. Likewise, the efficiency ofthe output element (be it a grating or other element, such as a beamsplitter or holographic optical element) can be adjusted by adding alayer of dielectric material to the outcoupling region. If outcouplingefficiency is too high, a high threshold current is required to lasebecause of the quantity of photons escaping the cavity. With areflective layer atop the outcoupling grating, some of the light isreflected and continues to oscillate within the cavity. This has theeffect of marginally decreasing the required current for lasing. Addinga dielectric layer (for example, nitride, or a dielectric stack, or evena reflective metallic layer) to the outcoupling location thereforecontrols outcoupling loss and decreases the required threshold current.

In any configuration of the present application where one or moregratings are located and superimposed on one another, the separategratings can be individually formed by conventional means (includingmultiple exposures and etches to form the pattern) or can be merged intoa single element using a holographic optical element (HOE). The opticalrequirements of the gratings can be calculated, and a HOE can bedesigned that accomplishes these required optical functions. Oncedesigned, such a HOE can be patterned for lithography, and can thereforebe fabricated in fewer process steps than it would take to create themultiple gratings separately. For example, multiple divergent beams canbe captured and coupled into a single fiber with HOEs.

In another embodiment, shown in FIG. 3, a reflecting surface is placedbeneath the outcoupling gratings. This surface 11 reflects the two lowerlobes of emitted light. This reflective layer can be made from ametallic substance or other material reflective to the necessarywavelengths of light, or it might be a reflective grating formed in thedevice, for example. By coupling the lower lobes back up and out of theOC grating, greater power is coupled out of the laser and may becaptured by a fiber or other device, such as a detector to monitor thelight produced by the device. A space 12 is shown in the substrate ofthe figure, but the same reflective surface can be placed with thesubstrate intact.

Another embodiment is shown in FIG. 4a, which shows a method to increasethe lateral width of the gain regions at the outcoupling grating whilemaintaining a single-transverse mode. This is accomplished by using asingle mode waveguide in the gain region that connects to a tapered gainregion. The taper angle is related to the divergence of the fundamentalmode of the single mode waveguide. In the preferred embodiment, thetapered regions have a laterally varying current contact, such as agaussian contact to stabilize the modes in the tapered device.

The embodiment shown in FIG. 4a has a tapered gain region 13. The gainregion in this sample has a straight portion as well. Different contactsare used in the preferred embodiment, pumping the different regions withincreasing current as necessary. The tapered gain region ensures awide-spatially-coherent mode, and avoids the restriction on the lateral(y) dimension imposed by the requirement of single lateral modeoperation. A wide lateral mode allows the width of the output beam to beset by the width of the grating. The grating area can take a desiredshape to match the needs of various applications. For example, circular,elliptical, or Gaussian beams can be produced.

FIG. 4b shows the tapered gain regions used with multiple crossedgrating outcoupled lasers. This embodiment shows two crossed GO (gratingoutcoupled) lasers at 90 degrees to one another. DBR gratings 7 y, 7 z,create the cavity as in other embodiments discussed above. The pumpedregions of the lasers in this variation have a flared section (labeledwith the f suffix), being wider closer to the OC gratings. In thisexample, one laser has an outcoupling grating that is circular in shape,while the other laser has an outcoupling grating that is rectangular.Using a tapered gain region, outcoupled beams of a greater range ofsizes can be produced, as the gain region can be made to whatever widthis necessary to accommodate (or fully take advantage of) the size of theOC grating. Tapered gain regions also increase the total amount ofemitted power from the device.

Another embodiment is shown in FIG. 5a. At one end of the laser, the DBRhas been replaced by a reflective end facet 7 y. There are still twogain region portions, separated by the OC grating in this embodiment.The other end of the laser has a DBR 7 yB, the period of whichdetermines the wavelength that will be stable in the cavity.

A top view of a crossed GO laser system is shown in FIG. 5b. In thisvariation, the laser running in the z-direction (horizontal in thefigure) has reflective end facets on both ends, and no DBRs forreflecting the light in the cavity. The y-direction laser (vertical inthe figure) has a DBR at one end, and a reflective facet at the otherend. Cleaved facet ends reduce the length of the device since the lengthof the DBR sections is omitted.

FIG. 5c shows a top view of a laser diode with cleaved facets at bothends. The length of the gain region is fixed by the reflectivity of theend facets or the DBRs.

FIG. 6 shows a laser with two reflective end facets and an OC gratingbetween the pumped regions. Beneath the OC grating is a reflector forreflecting the two downward directed beams back up toward the surface ofthe laser. Capturing the downward beams is useful to increaseefficiency.

FIG. 7a shows a GO laser system with four crossed lasers beingoutcoupled at the same spatial location. Distributed Bragg reflectorsare shown in this example at each end of the individual lasers. Eachlaser has its own OC grating superimposed on the other OC gratings tocreate one virtual grating. (Note that it is possible to overlap thegratings in different planes, having either curved waveguides or makingeach laser totally in a different plane than the others, but this is aless preferred embodiment. HOEs can be used in place of multiplegratings in such a situation.) FIG. 7b shows a close-up of theoutcoupling gratings for the four laser system. There are four gratings,each at a 45 degree angle to the adjacent gratings. The periods of thesegratings need not be the same, which allows the use of multiplewavelengths being outcoupled at the same location (assuming the lasersthemselves are fashioned to output different wavelengths). The periodsof the various gratings are labeled as Lz, Ly, Lzy, and Lyz.

The structure of the grating outcoupled laser allows for the high-levelintegration of electronic circuitry with the laser device. Manydifferent possible devices can be integrated with the GO laser of thepresent application, including control electronics and photonicintegrated circuits that can be placed on the chip with the laser;electronic integration includes higher level data management electronicssuch as serialization/deserialization, clock generation and recovery, opamps and analog-to-digital and digital-to-analog converters. The GOlaser geometry is also advantageous in that it allows for integrationwith optical waveguides and integration of light routing circuitry onthe chip. Such photonic integration allows optical interconnects betweencircuit elements, integration of optical isolators, wavelength tunablesections, optical modulators, waveguide couplers, waveguide switches,and simultaneous integration of multiple ones of these components.

FIG. 12 shows another embodiment of the present innovations. In thisvariation, a reflective coating 1202 (for example, a dielectric stack ora metal layer) is placed on top of the device, where the outcouplinggrating 1204 is located. This reflective coating causes the light to becoupled through the bottom of the device. (Note that it is often usefulto allow some light to escape the top of the device in this design, asthis light can be used to allow easier wafer level probe testing of thedevice.) If an outcoupling grating is used without such a reflectivecoating, there is of course light emitted both above and below thewaveguide due to reflection from the grating. The reflective coatingcauses the top emitted lobe of light to instead be emitted through thebottom of the device, so that substantially all light emitted from thecavity is coupled out through the substrate material, which is atransparent material, usually about 100 microns thick. The reflectivecoating can be metallic (such as gold) or it can be a dielectric stackfor better reflection. Generally, if the light is to be emitted out thebottom of the device, a high reflect (HR) coating is placed on top ofthe device to reflect the light downward. An anti-reflect (AR) coatingmay be added to the bottom of the device in this case. Alternatively, ifthe light is to be emitted out the top surface, the HR coating may beplaced on the bottom of the device, and an AR coating placed on top.

Causing the light to be emitted through the bottom of the device has theadvantage of allowing the heat sink to be placed on the top of thelaser, closer to the locations where heat is generated, increasingefficiency of heat sinking. In such a case, the device is preferablymounted “upside down”, with the reflective coating and DBRs beneath, thesubstrate on top, facing a fiber core for coupling, for example. Thisvariation helps feedback in the cavity and can decrease thresholdcurrent by 50% or more.

In FIG. 12, the laser device is shown with an outcoupling aperturebetween the DBRs. An outcoupling grating is shown in this example. Ontop of this grating is located a reflective coating that directsoutcoupled light down through the substrate.

The gain region or regions of the present innovations can be modified toprovide added functionality. FIG. 13 shows a DBR laser according to anembodiment of the present innovations. The gain region 1300 has multipleparts in this example, one on either side of an outcoupling grating. Onepart of the gain region is further split into two parts, a larger 1302and a smaller 1304 section. The smaller part, which can be used to moresensitively adjust the current supplied to the gain region, is used forseveral purposes. It can be used as a fine tuning device for thewavelength of the light in the cavity. By increasing or decreasing thecurrent, the wavelength can be slightly tuned to some degree. Thesmaller contact can also be used to modulate the signal generated in thecavity. By varying the supplied current over time, the intensity of theemitted light can be varied. This can be used to modulate the signal byadjusting the current over time to alter the intensity of light, andthus embed a signal in the emitted light. The smaller contact is thepreferred one to use for such modulation, since it will allow fastermodulation (due to lower capacitance, etc.).

FIG. 14 shows a side view of a DBR laser according to anotherembodiment. The laser has a cavity and two reflector gratings 7, one ateach end. Between the reflector gratings are the gain region 1402 andthe outcoupling grating 8.

The waveguide 3 in the laser may be made from multiple layers includingan active and guiding layer containing the junction between the p and ntype semiconductor materials. For example, this region might comprise asequence of one or more barrier layers alternating with quantum welllayers.

In the embodiment shown in the figure, the quantum wells 1404 are madethinner beneath the reflector gratings 7 and beneath the outcouplinggrating 8. This results in a less lossy, more transparent device. Thelarger bandgap in the thinner quantum well regions (i.e., in theunpumped regions) means less absorption of photons in the cavity becausehigher photon transition energy is required. This lowers internal loss,increases efficiency, and lowers the required threshold current for thedevice.

The quantum well thickness, by controlling the required transitionenergy, affects the wavelength of the photons that will lase in thecavity. This allows large scale tunability of the device duringfabrication by controlling quantum well formation.

Quantum wells are fabricated at different thicknesses using selectivegrowth of epitaxial layers. This selective growth phase of processingcan also be used to simultaneously improve the performance of integratedcomponents such as electro-absorption modulators, which benefit fromquantum well structures that can be made more transparent (or have ahigher photon transition energy). The integrated devices are fabricatedat the same time and during the same set of processes as the lasersthemselves, and selective growth can be used on them without significantprocess cost added.

FIG. 15 shows another embodiment. In this variation, afractional-spherical lens 1502 made of an optical polymer is formed onthe outcoupling grating 1504 (by a microjet process, for example). Thislens acts to make different wavelengths of light (which may exit the OCGat different angles than the normal, or from one another) converge to becoupled into a fiber mode, for example. Since the index of refraction ofthe material outside the lens is lower than the index of refraction ofthe lens, light rays passing through the lens to the lower indexmaterial will be refracted away from the normal to the surface of thelens at that point. Depending on the index difference, the refractioncan be great or small. This element can be used to allow design marginin the size of the outcoupling grating. With the added lens, theoutcoupling grating can be made larger, (50 microns, for example), whichis useful in high power applications. Normally, an outcoupling gratingof 10 micron size can outcouple light in the hundreds of milliwatt rangeor higher, but with larger outcoupling gratings light of higher power,well above a watt, can be outcoupled and confined to a fiber mode.

Other types of intermediate lens devices (gratings, HOEs, etc.) can alsobe used at this location to aid in coupling light from the outcouplinggrating into a fiber core (for example). Such devices can be made onchip, or be separate devices added after processing is complete.

FIG. 15 shows a sample embodiment of the device. This embodimentcomprises a DBR laser as described previously in this application, withoutcoupling grating 1504 located between the two reflector gratings1506. Gain regions 1508 are on either side of the OCG. A lens 1502 whichis partly spherical in shape is formed on the outcoupling grating.

Details and Alternatives for Electronic Device Design and Fabrication

The most straightforward way to achieve integration of the electronicand photonic device structures is simply to provide the appropriateepitaxial structures for both. This can be done, for example, by using astacked-epi structure, where the epitaxial layers for the electronicdevices are simply grown over the epitaxial layers for the lasers (andpossibly other photonic devices). In this class of embodiments, growthof the conventional epitaxial structures for transistors (e.g. alightly-doped GaAs layer adjacent to a more-heavily-doped AlGaAs layer,in a conventional HEMT structure) is simply performed after the growthof the quantum-well structures for the lasers. A masked etch can be usedto remove the electronic device layers from the laser locations, oralternatively a selective or masked growth process can be used.

An alternative embodiment, which is contemplated as advantageous forfuture use, is to share at least some parts of the epitaxial structuresfor both electronic and photonic components. This has the advantage ofreducing the thermal budget during epitaxial growth (and also reducingcost), but is more complex to implement.

A further alternative is to form layers for the photonics structuresover the epitaxial (or substrate) layers which are used for theelectronic devices.

Definitions

Following are short definitions of the usual meanings of some of thetechnical terms which are used in the present application. (However,those of ordinary skill will recognize whether the context requires adifferent meaning.) Additional definitions can be found in the standardtechnical dictionaries and journals.

Chirped Grating: a grating whose period varies over its area;

DBR: Distributed Bragg Reflector;

DFB: Distributed Feedback (Laser);

DWDM: Dense Wavelength Division Multiplexing;

GO: Grating Outcoupled (Laser);

HOE: Holographic Optical Element;

LED: Light Emitting Diode;

LD: Laser Diode

Modulate: any controlled variation of the signal, in intensity orfrequency or phase;

OCG:Outcoupling Grating;

VCSEL: Vertical Cavity Surface Emitting Laser;

Waveguide: a structure which guides the propagation of photons, usuallyby internal reflection.

According to a disclosed class of innovative embodiments, there isprovided: An electronic/photonic integrated circuit, comprising: aplurality of lasers having lateral cavities; at least one gratingpositioned to outcouple light from at least one said laser; at least onelateral waveguide which is optically coupled to be driven by at leastone said laser; and electronic circuitry including at least onetransistor which is formed from a body of material which is common to atleast one said laser.

For example, the body of material can ultimately be not continuousbetween said laser and said transistors.

For example, each said laser can have a cavity which is defined solelyby a distributed reflector.

For example, the semiconductor structure can be epitaxially grown on asubstrate which consists essentially of monocrystalline InP.

For example, the body of material can comprise multiple substantiallylattice-matched layers of III-V compound semiconductor material.

According to another disclosed class of innovative embodiments, there isprovided: An electronic/photonic integrated circuit, comprising: a lasercomprising at least one semiconductor gain volume, distributed reflectorgratings defining a laser cavity beam path through said gain volume, andan outcoupling grating which deflects a portion of the energy in saidbeam path out of the plane of said reflector gratings; and transistorswhich are electrically coupled to said laser; wherein said transistorsand said gain volume are formed in a single multilayer structure ofsemiconductor material.

According to another disclosed class of innovative embodiments, there isprovided: An electronic/photonic integrated circuit, comprising: atleast one GSE laser; and at least 10,000 transistors, formed in asemiconductor structure which is at least partly shared with said laser.

According to another disclosed class of innovative embodiments, there isprovided: A method for fabricating electronic/photonic integratedcircuits, comprising the actions of: forming transistors andlateral-cavity lasers in a single body of monocrystalline semiconductormaterial; and forming gratings which outcouple light from at some onesof said lasers, to thereby form grating-outcoupled surface-emittinglasers; forming electrical connections to said transistors and saidlasers; and forming lateral waveguides which provide opticalinterconnects to at least some ones of said lasers.

According to another disclosed class of innovative embodiments, there isprovided: A method for fabricating electronic/photonic integratedcircuits, comprising the actions of: forming transistors andlateral-cavity lasers in a single body of monocrystalline semiconductormaterial; forming gratings which outcouple light from at some ones ofsaid lasers, to thereby form grating-outcoupled surface-emitting lasers;said lasers also comprising distributed reflectors having a differentperiod than said gratings; and forming electrical connections to saidtransistors and said lasers.

Modifications and Variations

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given.

The device structures used for the electronic circuitry can be HEMTs, oralternatively HBTs, MESFETs, MOSFETs, gain diodes, or other structures.

In one class of embodiments, epitaxial layers are separately optimizedfor the optical emission structures and for the channel portions of thetransistor structures. In one such example, the semiconductor layerwhich overlies the quantum well(s) of the laser structures is used asthe body for a HEMT (or as the emitter for an emitter-down HBT), and thechannel (or base) and barrier (or collector) layers are successivelyformed thereover.

In an alternative class of embodiments, the SAME layer which is used forthe gain volume of the laser structures is used as a channel (or base,or more generally “conductivity-control”) volume for the activesemiconductor devices. This can be accomplished in several ways:

In one alternative, the laser structure can simply be a single-wellstructure.

In another alternative, a multi-quantum-well laser structure can be usedas the channel of a HEMT; even if inversion does not occur below the toplayer of the narrowest-bandgap material, switchable modulation of onelayer's conductivity is sufficient for operation of the device.

In another alternative, a disordering treatment (local transientheating, diffusion or neutral implant) can be applied to the deviceareas, to smoothe out the vertical changes in composition which providethe multiple quantum wells for the laser's gain volume.

The electronic circuitry is not limited to switching and drivercircuits, but can also include, for example, imagers, detectors, DRAMs(transistor with capacitor), SRAMs, gate arrays, microprocessors, oranything else normally done in silicon or in III-V technologies.

The above list of alternatives is not exhaustive, and various otherstandard techniques used for microfabrication of lasers and/orintegrated circuits can be used as well.

None of the description in the present application should be read asimplying that any particular element, step, or function is an essentialelement which must be included in the claim scope: THE SCOPE OF PATENTEDSUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none ofthese claims are intended to invoke paragraph six of 35 USC section 112unless the exact words “means for” are followed by a participle.

What is claimed is:
 1. An electronic/photonic integrated circuit,comprising: a plurality of lasers having lateral cavities; at least onegrating positioned to outcouple light from at least one said laser; atleast one lateral waveguide which is optically coupled to be driven byat least one said laser; and electronic circuitry including at least onetransistor which is formed from a body of material which is common to atleast one said laser.
 2. The integrated circuit of claim 1, wherein saidbody of material is ultimately not continuous between said laser andsaid transistors.
 3. The integrated circuit of claim 1, wherein eachsaid laser has a cavity which is defined solely by a distributedreflector.
 4. The integrated circuit of claim 1, wherein saidsemiconductor structure is epitaxially grown on a substrate whichconsists essentially of monocrystalline InP.
 5. The integrated circuitof claim 1, wherein said body of material comprises multiplesubstantially lattice-matched layers of III-V compound semiconductormaterial.
 6. An electronic/photonic integrated circuit, comprising: alaser comprising at least one semiconductor gain volume, distributedreflector gratings defining a laser cavity beam pat through said gainvolume, and an outcoupling grating which deflects a portion of theenergy in said beam path out of the plane of said reflector gratings;and transistors which are electrically coupled to said laser; whereinsaid transistors and said said volume are formed in a single multilayerstructure of semiconductor material.
 7. The integrated circuit of claim6, wherein said structure is not continuous between said laser and saidtransistors.
 8. The integrated circuit of claim 6, wherein saidsemiconductor structure is epitaxially grown on a substrate whichconsists essentially of monocrystalline InP.
 9. The integrated circuitof claim 6, wherein said body of material comprises multiplesubstantially lattice-matched layers of III-V compound semiconductormaterial.
 10. An electronic/photonic integrated circuit, comprising: atleast one GSE laser; and at least 10,000 transistors, formed in asemiconductor structure which is at least partly shared wit said laser.11. The integrated circuit of claim 10, wherein said semiconductorstructure is not continuous between said laser and said transistors. 12.The integrated circuit of claim 10, wherein each said laser has a cavitywhich is defined solely by a distributed reflector.
 13. The integratedcircuit of claim 10, wherein said semiconductor structure is epitaxiallygrown on a substrate which consists essentially of monocrystalline InP.14. The integrated circuit of claim 10, wherein said body of materialcomprises multiple substantially lattice-matched layers of III-Vcompound semiconductor material.